Induction heating apparatus

ABSTRACT

An induction heating or cooking apparatus comprises an inductive heating coil (L), means (CT) for sensing the current I s  of an AC supply applied to the heating coil (L), means (VR) for selecting the desired heat output of the apparatus, means (APC) for comparing the sensed current with in output of the selecting means (VR), and means (RL1) for varying a the voltage of the supply to the heating coil (L) in accordance with the value of an error signal output from the comparing means (APC). Thus, the temperature which the base of a cooking utensil (P) stood on the coil (L) reaches is independent of the material of the utensil.

BACKGROUND OF THE INVENTION

This invention relates to induction heating apparatus.

Cooking hobs are known which comprise one or more large induction coils,on which pans having an electrically inductive base can be stood. Inuse, a high frequency signal (in excess of 20 kHz) is applied to thecoil, which generates a magnetic field that induces eddy currents in thepan base. The base of the pan is not an ideal conductor, and thus theelectrical energy is dissipated as heat as current flows through the panbase. Thus, the heating effect is proportional to I² R, where I is thecurrent in the base of the pan and R is the electrical resistance of thepan.

The resistivity of the pan base depends on the material that it is madefrom. Thus, it will be appreciated that the temperature which the panbase reaches will be dependent on the material of the pan, with theobvious disadvantage that discrepancies will occur between the heatsetting, which has been selected by the user, and the actual heatdeveloped.

SUMMARY OF THE INVENTION

We have now devised an inductive heating apparatus which alleviates theabove-mentioned problem.

In accordance with this invention, there is provided an inductionheating apparatus comprising an inductive heating coil, means forsensing the current and/or voltage value of an a.c. supply applied tothe heating coil, means for selecting the desired heat output of theapparatus, means for comparing the sensed current and/or voltage valuewith an output of the selecting means, and means for varying a parameterof the supply to the heating coil in accordance with the value of anerror signal output from the comparing means.

In use, we have found that the heating coil of an inductive heatingapparatus acts rather like the primary winding of a transformer with thepan acting as a single shorted turn secondary winding. The heatingeffect in the base of the pan is proportional to I² R, where I is thecurrent in the base of the pan and R is the electrical resistance. Theheating effect in the base of the pan is also dependent on the depth ofpenetration of the magnetic field into the base, and this depth ofpenetration is inversely proportional to the coil frequency. Thus, itwill be appreciated that the heating effect at a given frequency can bedetermined by measuring the current and/or voltage at the coil primary,and that the heating effect can thus be varied by varying the current,voltage or frequency value applied to the coil.

In one embodiment, the varying means is arranged to vary the drivefrequency which is applied to the coil, in order to vary the depth ofthe penetration of the magnetic field into the pan base, so that theheating effect is correspondingly varied.

Preferably the heating coil forms part of a tuned circuit, which ispreferably arranged to oscillate at its resonant frequency, in order tomaximise the voltage across the coil, and hence maximise its efficiency.

Preferably the resonant frequency of the heating coil is varied as thedrive frequency is varied, by varying the impedance of the coil and/orby varying the capacitance of the tuned circuit.

Preferably the impedance and/or capacitance is varied by respectivelyswitching inductors and capacitors into or out of the tuned circuit.

A disadvantage of varying the operating frequency of the coil is thatbeating or heterodyning can occur if there is more than one heating coilin an inductive heating apparatus. This beating or heterodyning occurswhen the coils operate at a different frequency, thereby causing a thirdfrequency of a value which is equal to the difference in the coilfrequencies. Often, this frequency will be less than 16 kHz, with theresult that it is audible and annoying to users.

Thus, it is preferable that the coil operates at a fixed frequency inorder to avoid the problems of heterodyning.

Thus, in an alternative embodiment the varying means is arranged to varythe value of the current applied to the coil by varying its number ofturns, and from this it will be appreciated that a different voltage andcurrent are induced in the secondary, with a correspondingly differentheating effect.

A disadvantage of varying the number of turns of the coil is that theresonant frequency varies. Thus, in order to compensate for this thevarying means is preferably arranged to increase the capacitance of thetuned circuit when the number of turns of the coil is reduced andvice-versa, so that the multiple of the inductance and capacitance ofthe tuned circuit remains the same, thereby keeping the resonantfrequency constant.

In an alternative preferred embodiment, the varying means is preferablyarranged to vary the voltage applied to the coil, in order tocorrespondingly vary the voltage induced in the pan or other cookingutensil.

The voltage applied to the coil can only be varied within theconstraints of the supply voltage, and thus the heating coil ispreferably connected across the secondary of a transformer, the varyingmeans being arranged to vary the voltage across the coil by varying thenumber of turns of the transformer primary or secondary.

Preferably the heating coil forms a part of a tuned circuit, which ispreferably arranged to oscillate at its resonant frequency. Thus, thevarying means is preferably only arranged to vary the number of turns ofthe transformer primary, because a variation in the number of turns onthe secondary would affect the resonant frequency of the tuned circuitconnected thereto.

These and other objects, features and advantages of the presentinvention will be clearly understood through consideration of thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The varying means may also be arranged to vary the voltage across thecoil by varying the value of the supply to the transformer primary.

Embodiments of this invention will now be described by way of examplesonly and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a conventional induction heatingapparatus;

FIG. 2 is a schematic diagram of a first embodiment of induction heatingapparatus in accordance with this invention;

FIG. 3 is a schematic diagram of a second embodiment of inductionheating apparatus in accordance with this invention; and

FIG. 4 is a schematic diagram of a third embodiment of induction heatingapparatus in accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, there is shown a conventionalinduction heating apparatus comprising a pair of high frequency powertransistors SW1,SW2 connected in series across the supply. A pair ofcapacitors C1,C2 are also connected in series across the supply.

An inductive heating coil L and a capacitor C are connected in seriesbetween two points which are respectively disposed at the connectionpoint between the transistors SW1,SW2 and at the connection pointbetween the capacitors C1,C2. The transistors SW1,SW2 are controlled bya high frequency driver circuit DR.

In use, a pan P is stood on the inductive heating coil L and the heatingapparatus is energised. Initially C is discharged, however, when SW1 isclosed C and C2 are charged from the supply through L. When C is fullycharged SW1 opens and SW2 closes, whereupon C discharges through L andC1. This cycle is repeated continuously, thereby providing analternating magnetic field in induction heating coil L.

SW1 and SW2 are switched at or near the resonant frequency of the tunedcircuit LC, so that losses are kept to a minimum. The resonant frequencyof the tuned circuit LC is defined as

    f=1/(2×π×√(L×C))

A disadvantage of this arrangement is that the heating effect in the panP is proportional to I² R, where R is the electrical resistance of thepan base and I is the current flowing through the base. Thus, it will beappreciated if the temperature control dial is set to provide apredetermined current and/or voltage to the coil, then the temperatureproduced will actually depend on the material of the pan base.

In order to overcome this problem, the conventional circuit can bemodified in accordance with this invention, as shown in FIG. 2 of thedrawings. The arrangement of the circuit of FIG. 2 is similar to that ofFIG. 1, and like parts are given like reference numerals. The maindifference between the two circuits is that the capacitor of the tunedcircuit is replaced by a bank of capacitors C_(a),C_(b),C_(c) connectedin parallel. Each of the capacitors C_(a),C_(b),C_(c), is connected inseries with the switched contacts of respective relays RL1,RL2,RL3. Acurrent sensing coil CT monitors the current flowing through the heatingcoil L.

A potentiometer VR for selecting the heat setting of the hob isconnected to an automatic power control circuit APC, which controls thetransistor driver circuit DR. The power control circuit APC eithermeasures the voltage or current from the potentiometer VR, in order todetermine the desired heat setting. The current sensing coil isconnected to the power control circuit APC. The energising coils of therelays RL1,RL2,RL3 are connected between the positive supply and amicroprocessor M, which is controlled by the power control circuit APC.

The circuit operates in the same way as the circuit of FIG. 1, with thecapacitors C_(a),C_(b),C_(c), charging and discharging through the coilL at a frequency near resonance.

The power control circuit APC receives a voltage from the currenttransformer CT, which is proportional to the voltage across the coil L.The coil L acts as the primary of a transformer, with the pan acting asa single, shorted turn secondary winding. The voltage V₂ across thissingled shorted turn secondary winding is equal to the voltage V₁ acrossthe primary (i.e. coil L) multiplied by the turns ratio (N₂ /N₁) of theeffective transformer. The voltage V₂ across the secondary isproportional to the heating effect in the pan base, and thus it will beappreciated that the output of the current sensing coil CT also isproportional to the heating effect.

The power control circuit APC compares the output of the current sensingcoil CT with the power setting selected by potentiometer VR, andproduces an error signal. This error signal is fed to the microprocessorM, which determines whether the drive frequency needs to be adjusted,since an increase in frequency will produce a decrease in currentpenetration in the pan base, and a corresponding lower heating effect,and vice-versa.

The resonant frequency is controlled by switching selected capacitorsC_(a),C_(b),C_(c) in the capacitor bank into or out of the tuned circuitusing the relays RL1,RL2,RL3.

A disadvantage of this system is that a large number of capacitors arerequired in the capacitor bank if a fine control of the frequency, andhence of the power, is to be provided.

Another disadvantage is that in hobs having two coils, each coil will berunning at a different frequency, with the result that their frequencieswill interact or heterodyne, thereby producing an audible whine at afrequency which is equal to the difference between the two coilfrequencies. This audible whine will constitute a nuisance to theequipment operator.

Referring to FIG. 3 of the drawings, there is shown an alternativeembodiment of induction heating apparatus, and like parts are given likereference numerals. In this embodiment, the heating coil L comprises anumber of taps on its windings which are respectively connected to oneside of the coil L through respective switches e.g. SWL.

It will be appreciated from the formula V₂ =V₁ ×N₂ /N₁ that theeffective voltage V₂ developed across the pan base is dependent upon theratio N₂ /N₁ of the windings. Thus, the heating effect developed in thepan P can be varied by switching selected switches e.g. SWL, so as tovary the number turns N₁ on the coil L.

A disadvantage of this arrangement is that the impedance of the coil Lchanges as its turns are varied, which affects the resonant frequency ofthe tuned circuit. However, this disadvantage can be overcome byswitching capacitors C_(a),C_(b),C_(c) in the capacitor bank intocircuit as the turns of the coil L are shorted, and vice-versa.

Thus, the circuit of FIG. 3 can be operated at a fixed frequency closeto its resonant frequency. However, the circuit still suffers from thedrawback that a large number of capacitors are required in the capacitorbank to achieve fine control. Similarly, a large number of coiltrappings are also required.

Referring to FIG. 4 of the drawings, there is shown a preferredembodiment of induction heating apparatus and like parts are given likereference numerals. In this embodiment, a so-called auto-transformer isconnected in place of the coil L. An auto-transformer is a transformerin which the secondary winding comprises a tapped section of the primarywinding. The heating coil L is connected in series with a capacitoracross the secondary winding of the auto-transformer T.

The voltage V₂ across the secondary of the auto-transformer T isproportional to the voltage V₁ across its primary times its turns ratio.

When current flows through the auto-transformer T it also flows throughthe heating coil L and the capacitor C, which are connected across thesecondary windings. When the capacitor C is fully charged SW1 opens andSW2 closes, so that C discharges through the heating coil L and throughthe auto-transformer. It will be appreciated that at this point thecurrent is flowing in the reverse direction through the heating coil L.SW1 and SW2 are controlled so that the cycle repeats at the resonantfrequency of the heating coil L and capacitor C.

The automatic power control circuit APC indirectly senses the currentI_(s) flowing through the heating coil L, by sensing the current flowingthrough the primary winding of the auto-transformer T. The power controlcircuit APC compares the sensed current with the setting produced by thepotentiometer VR and produces an output error signal, which is fed tothe microprocessor M. If the error signal is demanding more power, i.e.the signal magnitude at the potentiometer VR is larger than the signalmagnitude from the current sensing coil CT, the power control circuitAPC automatically increases the voltage on the supply rail +V until bothsignals are equal

It will be appreciated that the voltage V₁ across the primary of theauto-transformer T will rise if the supply is raised, and thatcorrespondingly more voltage will be developed across the coil L,thereby increasing the power delivered to the pan base.

If the power control circuit APC raises the supply voltage to a maximumand still cannot get enough power into the pan, the microprocessor Mdetects that not enough power is achieved and switches a relay RL1,which effectively reduces the number of turns N1 on the primary of theauto-transformer T, so that the voltage V₂ on the secondary increasesaccording to the formula

    V.sub.2 =V.sub.1 ×N.sub.2 /N.sub.1.

The resistance of the base of the pan remains constant as does thefrequency of operation and depth of penetration into the pan base. Thus,the power into the base increases as the secondary voltage V₂ increases.This technique solves the pan-to-pan power variations very economicallybecause only one relay is needed and no coil retuning is required.

The auto-transformer coil T automatically matches the impedance of thetuned circuit LC to the switching circuit, so that the power switchesalways switch within a known band of current values, irrespective of thetype of pan material. This means that less expensive power switches canbe used.

The switching current is transformed by the auto-transformer into thetuned circuit LC by the factor N₁ /N₂, and hence there is a highercurrent through the coil L than conventional systems, with acorrespondingly higher depth of magnetic field penetration into the panbase.

The power to the pan is not varied by varying the frequency and thus theproblem of beating or heterodyning is avoided. While the preferredembodiment of the invention have been shown and described, it will beapparent to those skilled in the art that changes and modifications maybe made therein without de-Darting from the spirit of the invention, thescope of which is defined by the appended claims.

What is claimed is:
 1. An induction heating apparatus comprising:anauto-transformer; an inductive heating coil and a capacitance connectedtogether to form a tuned circuit arranged to oscillate at a fixedfrequency, the tuned circuit being connected across a secondary tappingof a winding of the auto-transformer; means for sensing one of a currentvalue and voltage value of a power supply connected across theauto-transformer; means for selecting a desired heat output of theapparatus; means for comparing the sensed current or voltage value to aheat output selected with the selecting means; and means for varying thevoltage of the supply to the inductive heating coil, in accordance withthe value of an error signal output from the comparing means, by varyingthe number of turns of the winding of the auto-transformer, across whicha supply voltage is applied.
 2. An induction heating apparatus asclaimed in claim 1, in which the varying means is also arranged to varythe voltage value of the supply to the auto-transformer.